Kaolin is a naturally occurring, relatively fine, white clay mineral which may be generally described as a hydrated aluminum silicate. Kaolin has long been used as a coating and filler pigment for paper. Use of kaolin as a coating pigment improves various properties, such as brightness, color, gloss, smoothness, light scattering ability, uniformity of appearance and printability of a paper. As a filler, kaolin is used to extend fiber and reduce cost and improve opacity, brightness and other desirable characteristics. The chemically inert kaolin also possesses desirable low viscosity at high solids content when suspended in an aqueous medium with a small pH adjustment and dispersant additives.
Opacity is one of the most desirable optical properties of pigment coated paper and is directly related to the light scattering ability of the pigment. Light scattering ability of the pigment can be estimated by the light scattering coefficient using the Kubelka-Munk equation as described in TAPPI 1978, Vol. 61, No. 6, pages 78-80. Light scattering is a direct function of the index of refraction of the mineral. Visible light travels through media of different refractive indices at different speeds; faster in a medium with a lower refractive index. As light travels between two media of different refractive indices, the light will change velocity and bend or change direction. Several changes of media and several bendings of incident light will create "scattered" light.
Larger differences in refractive indices between materials will cause more severe distortions of the incident light. Minerals with higher refractive indices will scatter light more, thus improving opacity, than those of lower refractive indices. Titania (TiO.sub.2), depending on its crystal structure (rutile or anatase), has a refractive index of 2.55 to 2.7. Kaolin has a refractive index of 1.55. Therefore, a titania pigment will yield a coated paper having greater opacity than a kaolin pigment coated paper at the same coat weight, or lower amounts of titania may be used to match the kaolin opacity. TiO.sub.2, however, is an expensive pigment. Therefore, there is a need to develop an opacifying pigment from inexpensive raw material, such as kaolin, that can be used in place of the more expensive TiO.sub.2.
Scattering of light by a single pigment can be improved by increasing the number of mineral to air interfaces. The refractive index of air is 1.0 and that of kaolin is 1.55. Increasing the number of times light passes through two media of significantly different refractive indices results in higher chances for separation and diversion of the incident light. The distance traveled in each medium, however, must be about one half to one full wavelength of visible light (approx. 0.2 to 0.5 microns). One can then conclude that arranging the mineral particles in such a way to create 0.2 to 0.5 micron pores (voids) will increase the light scattering ability o pigment, and subsequently the opacity of paper coated with the pigment. Pore size and pore volume can be measured by mercury intrusion as described in the American Society for Testing and Materials (ASTM) method D 4284-92.
In addition to opacity, ink receptivity is a desired characteristic in many coated paper applications. Ink receptivity is the ability of a coating to absorb ink. An optimum ink receptivity is necessary to achieve an efficient transfer of ink to the paper during printing. Pigments created with mineral particles arranged to create pores for light scattering will also possess sufficient volume and capillary action in the open pores to absorb ink, thereby increasing the ink receptivity of the coated paper. The ink receptivity can be measured by applying the K & N ink to the coated samples for 2 minutes and then wiping off the excess ink. The brightness is measured before applying and after removing the ink. The percentage decrease in brightness is directly proportional to ink receptivity, that is, the larger the decrease in brightness, the more receptive the sample to application of ink. This test is described by J. C. Rice, Varnishing Characteristics of Coating Clays and Pigments, TAPPI, Vol. 39 (1), pp. 43-45, and in TAPPI Routine Control Method RC 19 (referenced on page 44 of the preceding article).
The aggregated or bulking pigment is also important for producing lightweight coated paper. Because of its highly opacifying nature, much smaller amounts of aggregated pigment are required to coat paper compared to a conventional pigment, thereby reducing the weight of the paper. An additional benefit of the lightweight coated paper is the reduced cost of postage, handling and shipping.
There are two primary methods for producing aggregated or structured kaolin clay pigments: (1) thermal and (2) chemical. Thermally structured kaolin clay is also called calcined clay which is produced by calcining the kaolin clay to temperatures above 900.degree. C. Specific examples of calcined kaolin are described in Fanselow et al. U.S. Pat. No. 3,586,523.
Several methods have been used to produce chemically aggregated kaolin products. Shi et al. U.S. Pat. No. 5,089,056 discloses the production of chemically modified pigments by hydrothermally reacting kaolin clay with sodium or potassium hydroxide. In this system, the reactants are heated in a closed vessel at temperatures from 150.degree. C. to 200.degree. C. The products are defined as extremely stable aggregates which incorporate light scattering sites.
Wason et al. U.S. Pat. No. 4,812,299 describes the production of synthetic alkali metal aluminosilicates by hydrothermally reacting kaolin clay with alkali metal silicate. In this system, the pressure ranges from about 50 to 360 pounds per square inch (psi) and temperature ranges from 140.degree. C. to 250.degree. C. The products after filtration and spray drying are identified as structured agglomerates formed from the integration of altered kaolin platelets with amorphous alkali metal silicate base-kaolin reaction products. These products are found to be useful as functional fillers, as TiO2 and silica extenders, or as reinforcing agents for paper, paint, rubber and plastics.
U.S. Pat. No. 5,203,918 to C. A. Rice discloses a relatively low temperature (room temperature) process of forming an aggregated pigment from kaolin by intermixing the kaolin slurry with 10-20% alum by weight of kaolin and 15-30% sodium silicate, followed by filtering and drying the slurry. The products are characterized as having a coarser particle size distribution and higher pore volume compared to the starting kaolin and are useful in coating and filling of papers.
Demmel U.S. Pat. Nos. 5,190,902 and 5,288,739, disclose the mixture of kaolin clay particles either at a very low pH (1.0 to 3.0) or a high pH (10.0 to 14.0) with ammonium phosphate compounds in a concentration of 2 to 20% by weight, followed by spray drying and calcining at temperatures between 538.degree. C. and 1,066.degree. C. The Demmel disclosures, however, are related to the production of a different product, namely attrition-resistant binder particles with vitreous qualities for binding catalyst particles into microspheroids used in fluid catalytic cracking processes. These abrasive, attrition-resistant binder particles are tough and not amenable for application to paper as a coating pigment because of their abrasion characteristics. In addition, the abrasive, attrition-resistant binder particles prepared according to the Demmel disclosures are large (approximately 65 microns in diameter) and are unsuitable for paper coating applications in which particle diameters below 45 microns are desirable.
What is needed are improved chemically aggregated kaolin clay pigments and a process for making such pigments. This method should avoid the use of calcining temperatures and should produce a chemically aggregated kaolin clay pigment with improved paper coating performance, ink receptivity, light scattering ability, porosity, and gloss compared to the starting pigment.